The present invention relates to a hydraulic drive unit for driving a hydraulic motor which is utilized as a traveling device in a construction machine. Particularly, the invention is concerned with a hydraulic drive unit capable of preventing the occurrence of cavitation while a hydraulic motor is OFF.
As this type of a hydraulic drive unit there is known, for example, such a hydraulic circuit as shown in FIG. 17.
In this hydraulic circuit, a hydraulic motor M is connected to an oil pressure source comprising a pump and a tank via a pair of main circuits 1 and 2, a counter-balancing valve 3 disposed halfway of the main circuits 1 and 2, and ports P1 and P2. Further, a return circuit 64 is disposed between the two main circuits 1 and 2, a pair of check valves 5 and 6 are disposed halfway of the return circuit 64 so as to be capable of being opened and closed, and a low pressure circuit 7 is connected to the return circuit 64 at a position between the two check valves 5 and 6. A leakage circuit from the motor M is connected to the low pressure circuit 7, which in turn is connected to the tank side via tank ports T1 and T2.
For rotating the hydraulic motor M in a forward direction for example, the pump port P1 is connected to the pump side and the other pump port P2 connected to the tank side through a change-over valve (not shown), allowing an oil pressure to be fed from the pump port P1. At this time, with a pilot pressure, the counter-balancing valve 3 changes over to its left-hand position, whereby an oil pressure is fed from the main circuit 1 to the hydraulic motor M, causing the motor to rotate in the forward direction. Return oil from the hydraulic motor M is returned to the tank side via the other main circuit 2, counter-balancing valve 3, and pump port P2.
When the change-over valve is changed over to its neutral position, the pilot pressure is extinguished, the counter-balancing valve 3 returns to its neutral position, and hence the supply of the pressure oil is stopped, with the result that the hydraulic motor M turns OFF.
However, at the beginning of turning OFF of the hydraulic motor M the motor rotates by the force of inertia. The hydraulic motor M sucks in oil in the main circuit 1 and discharges the oil into the other main circuit 2, that is, performs a so-called pumping operation. Thus, the main circuit 1 becomes negative in pressure, the hydraulic motor M sucks in air in the main circuit 1, and cavitation occurs, thereby generating a low-noise for the hydraulic motor M. Therefore, for preventing the occurrence of cavitation caused by vacuum action, oil is sucked in from the tank side by means of the low pressure circuit 7 and a hydraulic operating oil is fed to the hydraulic motor M via the return circuit 64, check valve 5 and main circuit 1.
In the above conventional hydraulic drive unit, when the hydraulic motor turns OFF, oil is supplied from the low pressure circuit 7 for preventing the occurrence of cavitation. In this connection, the low pressure circuit 7 is required to have a pressure and a flow rate both sufficient to effect the supply of oil.
However, since the low pressure circuit is a drain line, it is impossible to set its pressure so high, and for ensuring the required oil pressure and flow rate it is necessary to alter not only the hydraulic motor but also the circuit and the entire system used. However, when the hydraulic drive unit is used for a traveling device in a small-sized construction machine for example, it is impossible to alter even the circuit and system used in the machine body and it is the present situation that the improvement in deceleration feeling of the hydraulic motor and the prevention of low-noise occurrence due to cavitation are not effected to a satisfactory extent.
Accordingly, it is an object of the present invention to provide a hydraulic drive unit which, without the need of greatly altering the circuit configuration and the entire system, can prevent the occurrence of cavitation when a hydraulic motor is OFF and as a consequence thereof can also attain the improvement in the deceleration feeling of the hydraulic motor and the prevention of low-noise occurrence.
For achieving the above-mentioned object, according to one means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor; a counter-balancing valve disposed halfway of the main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of discharge passages and a pair of return passages, both of which are adapted to be opened and closed for the main circuits, a pair of first check valves disposed halfway of the discharge passages, a pair of orifices disposed halfway of the return passages, and a pair of branch passages which are connected to the discharge passages on upstream sides of the first check valves and selectively connected to the by-pass circuit; return circuits each connected to the main circuits in a position therebetween; a pair of second check valves disposed halfway of the return circuits; and a by-pass circuit connected to the return circuits in a position between the second check valves.
According to a still further means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor and having first main circuits and second main circuits which are in parallel with each other; first check valves disposed halfway of the first main circuits; a counter-balancing valve disposed halfway of the second main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of branch passages selectively opening and closing the second main circuits for the first by-pass circuit, a pair of return passages opened and closed for the second main circuits, and a pair of orifices disposed halfway of the return passages; return circuits each connected to the main circuits in a position there between; a pair of second check valves disposed halfway of the return circuits; and a first by-pass circuit connected to the return circuits in a position between the second check valves.
According to a still further means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor and having first main circuits and second main circuits which are in parallel with each other; first check valves disposed halfway of the first main circuits; a counter-balancing valve disposed halfway of the second main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of return passages opened and closed for the second main circuits, and a pair of orifices disposed halfway of the return passages; return circuits each connected to the main circuits in a position therebetween; a pair of second check valves disposed halfway of the return circuits; a first by-pass circuit connected to the return circuits in a position between the second check valves; second by-pass circuits connected to the main circuits in a position therebetween, in parallel with the return circuits; and a change-over valve disposed halfway of the second by-pass circuits and at the same time of changing-over thereof, selectively opening and closing the first by-pass circuit to one of the second by-pass circuits.
According to a still further means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor; a counter-balancing valve disposed halfway of the main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of discharge passages and a pair of return passages, both of which are adapted to be opened and closed for the main circuits, a pair of first check valves disposed halfway of the discharge passages, a pair of orifices disposed halfway of the return passages, and a pair of branch passages which are connected to the discharge passages on upstream sides of the first check valves and are selectively connected to the by-pass circuit; return circuits each connected to the main circuits in a position therebetween; a high pressure selection valve disposed halfway of the return circuits so as to be capable of being changed over from one position to another; and a by-pass circuit connected to the return circuits on a downstream side of the high pressure valve so as to be capable of being changed over from one position to another.
Preferably, auxiliary passages which communicate with a brake cylinder-side circuit so as to be capable of being opened and closed are connected to the branch passages of the counter-balancing valve.
Preferably, the counter-balancing valve consists of a spool type valve body inserted slidably into a valve hole of a valve body, said valve hole being provided with a land groove having a suitable width which opens and closes the by-pass circuit and the branch circuits in accordance with a stroke of the counter-balancing valve.